Efficiently separating solids from liquids is a widespread technical problem. One of the most practical and robust methods of achieving this remains the use of a sieve, or screen, to sift the solids from the mixture of liquid and solid.
When drilling for oil and/or gas, synthetic drilling fluids, or muds, are used. As these muds are relatively expensive to manufacture, once used they are typically recovered in a process including sifting rock, shale and other debris from the mud. This involves the use of a so-called shaker which has fitted, one or more sifting screens, made up of a screen frame with one or more sheets of woven wire mesh, or screen, stretched over and secured to it. In use, the shaker vibrates the sifting screen or screens, to aid the sifting process.
To be able to withstand the rigours of this sifting process, sifting screens must have a certain rigidity and be very hard-wearing. This has resulted in a design of sifting screens having a screen frame which has a plurality of reinforcing “ribs”. A typical design of a screen frame is rectangular comprising an outer rectangular perimeter with each side connected to its opposing side by a plurality of ribs. Such a design results in a plurality of rectangular openings. Typically the screen is attached not only to the rectangular perimeter but also to the ribs, to provide better adhesion of the screen to the frame and prolonging its lifetime.
In view of the fact that sifting screens are man-handled into position, such screen frames have for some time been made from plastics material to reduce weight. A typical design of plastic screen frame is reinforced by including a metal wire structure, embedded within the plastics rectangular perimeter and rib arrangement.
However, it has been found that such wires can lack the required stiffness, especially when extending between longer distances for large screen frames, and sag under gravity reducing their effectiveness as reinforcing structures.
It has been proposed, in e.g. GB 2461725, to use strengthening ribs between the upper and lower arrays of wires to improve the overall rigidity of the screen cage and frame. However using such ribs requires modification of the manufacturing process and associated tooling and increases material costs and complexity.
Thus further ways to improve rigidity of such screen frames without introducing significant weight to the screen frame would be highly desirable.